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This is a followup to my post last week about getting sent a mismatched stem and handlebar from an online shop. While the stem is no good to me and will be returned, the Ritchey Logic anatomical handlebars seem decent for the £10 (about $16 US) they cost on clearance.

While looking through my bike-bits drawer, I realized I had an unused pair of aluminum 31.8 to 25.4 shims. Placing these on the handlebar, they are a tight but almost perfect fit. They are a hair small, literally - if I press them to the bar with finger strength, they will stick in place, gripping the bar, with only a hair's gap visible between the handlebar and the center of the curve of the shim. I suspect that with the added pressure from a stem clamp, they will fit perfectly.

Does this sound like an OK thing to do? I could also try sanding out the inside of the shims a bit, but it almost seems unnecessary...

The bars are aluminum, not that high-end, so I figure the tubing won't be too delicate or thin. The shims are also aluminum.

Bikes: too many bikes from 1967 10s (5x2)Frejus to a Sumitomo Ti/Chorus aluminum 10s (10x2), plus one non-susp mtn bike I use as my commuter

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The diameter of the clamp area of the stems needs to be a fairly precise match to the diameter of the clamped bar, or as in this case the bar and shim. If you try to clamp this, something will have to distort and you risk cracking the clamp plate or the handlebar.

I've seen damage with much closer fitting parts, yours is definitely too far off. But you should be able to find a correct shim to match. You could sand or file .6mm of either the ID or OD of the shim. It's springy enough to flex to a new curvature, so it doesn't rally matter if you file the ID or OD, but you have to get the wall thickness to 2.9mm or very close to that to work.

I see - that makes sense, although I'd be worried that by sanding I might introduce uneven thickness that might be worse than uniform slight overtightness. There seems to be some wiggle room though, for example, 25.8 bars that work in both 25.4 and 26.0mm stems, isn't there?